A system is known in which the exhaust passageway of an internal combustion engine is provided with a NOx storage-reduction catalyst (hereinafter referred to as an “NSR catalyst”), an example of which is disclosed in JP-A-2001-271679. The NSR catalyst serves the function of adsorbing the nitrogen oxides (NOx) contained in combustion gases discharged from the internal combustion engine, as well as serving the catalytic function of purifying the NOx, hydrocarbons (HC), and the like. When the internal combustion engine is being operated at a lean air-fuel ratio, a NOx-rich exhaust gas is discharged. Thus, the NSR catalyst adsorbs this NOx, thereby preventing the NOx from flowing past the catalyst.
The NOx adsorbed by the NSR catalyst is purified at a particular timing. For instance, the above conventional system is designed to cause a rich-spike by temporarily discharging unburnt gas components from the internal combustion engine. This causes reactions within the catalyst between the NOx stored by the catalyst and the discharged unburnt gas components.
When the rich-spike causes the internal combustion engine to discharge a large amount of unburnt gas components, the exhaust gas flowing past the NSR catalyst becomes stoichiometric as long as there remains, in the catalyst, NOx to be reduced by the unburnt gas components. After the NOx adsorbed by the catalyst has all been reduced, the exhaust gas becomes richer because some unburnt gas components start to flow past the catalyst. Therefore, the above conventional system is designed to detect such an exhaust gas change into a rich one, which occurs downstream of the catalyst, by monitoring oxygen concentration or nitrogen oxide concentration, and the timing of that detection is followed by termination of the rich-spike. This prevents the rich-spike from being caused in an excessive manner, thereby also preventing deterioration of fuel consumption.    Patent Document 1: JP-A-2001-271679    Patent Document 2: JP-A-2009-114879